1. Field of the Invention
This invention relates to phased arrays and more particularly relates to phase/amplitude control.
2. Description of the Related Art
Phased array systems employ an array of antennas to permit directional signal reception and/or transmission. The array may be one-, two-, or three-dimensional. Arrays operate on a principle similar to that of a diffraction grating, in which the constructive and destructive interference of evenly spaced waveforms cause a signal of interest arriving from one angular direction to be strengthened, while signals from other angular directions are attenuated. By separately controlling the phase and the amplitude of the signal at each antenna of the phased array, the angular direction of travel of the signal of interest may be selectively enhanced and undesired signals may be excluded.
For example, consider a simple linear array of antennas spaced evenly a distance d apart, receiving/transmitting a signal of wavelength λ at an angle θ from the vertical. The time of arrival of the signal to/from each antenna will be successively delayed, manifesting itself as a phase shift of (2πd/λ)sin θ modulo 2π. By incrementally shifting the phase of the signal to/from each successive antenna by that amount, the combined signal to/from the array will be strengthened in the direction of angle θ.
Existing circuitry to shift the phase of a radio frequency (“RF”) signal by a variable amount is expensive, bulky, and not well-suited to integration on a chip. Because the circuitry must be replicated for each antenna in the phased array, the overall system cost becomes prohibitive for many applications.
Another problem is that phase shifting alone may not be sufficient to isolate the signal of interest. Selective amplitude control at each antenna may also be required, adding the cost of a wideband variable gain amplifier to each antenna branch. Amplitude control enables array pattern sidelobe control, which allows further reduction of interference with other undesired transmitters or receivers relative to phase-only beamforming.